1. Field of the Invention
The present invention relates to an apparatus for detecting a rotating condition of an article, and more particularly to an apparatus for detecting a rotating condition of an article by applying a lattice pattern formed in a predetermined pattern to the article, applying a light beam from a laser to the lattice pattern and detecting the pattern by a diffraction light from the lattice pattern.
2. Description of the Prior Art
Rotary encoder, rotation meter and linear scale have been known as the apparatus for detecting the rotating condition of the article. Recently, a demand for an apparatus which detects the rotating motion of the article at a high precision has been increasing. The rotary encoder uses a method of optically detecting the rotating condition of the rotating article.
In the prior art photoelectric rotary encoder, a main scale having equi-spaced light transmission areas and light blocking areas formed in a periphery of a disk coupled to a rotating shaft and a stationary index scale having light transmission areas and light blocking areas at the same pitch as those of the main scale are opposingly arranged between light projection means and light sensing means to form an index scale system. In this system, as the main scale is rotated, signals corresponding to the spacing between the light transmission area and the light blocking area of the scale are produced and the signals are frequency-analyzed or accumulated after reshaping to detect a rotation angle, variation in a rotation speed or a rotation direction.
In such a rotary encoder, the narrower the spacing between the light transmission area and the light blocking area of the scale is, the higher is the detection precision. However, as the spacing is reduced, an S/N ratio of the output signal from the photosensing means is decreased and the detection precision is lowered by virtue of diffraction of the light. If the total number of the light transmission areas and the light blocking areas of the main scale is fixed and the spacing between the light transmission area and the light blocking area is increased to such an extent that the affect by the diffracted light is avoided, a diameter of the main scale must be increased and a thickness must also be increased. As a result, the apparatus is of big size and a load to the rotating article under test increases.